1. Field of the Invention
The present invention relates generally to the preparation of alloy materials capable of hydrogen storage and suitable for use in rechargeable electrochemical cells. More particularly, this invention relates to master alloys useful in preparing metal alloys used as electrodes in rechargeable electrochemical batteries or cells. More particularly, the invention relates to the preparation of alloys containing vanadium nickel and chromium and other metals suitable for use as hydrogen storage electrodes.
2. Description of Related Art
Secondary cells using rechargeable hydrogen storage negative electrodes are now well known in the battery art. The reactions which take place at both the positive electrode and the negative electrode reversibly store hydrogen electrochemically. The advantages of such cells are well known, making such hydrogen storage cells particularly desirable and suitable for many commercial applications.
In U.S. Pat. No. 4,948,423, Fetcenko et al., there are disclosed numerous techniques by which hydrogen storage alloy material can be formed including vacuum induction melting in high density, high purity graphite crucibles. Many of those techniques employ high temperature melts but those techniques generally suffer for a variety of reasons. For example, while some processes are operative on small laboratory and pilot scales, when large scale operations are attempted, they become too costly to be commercially effective.
While consumable arc melting has been used, it also has difficulties which are burdensome including the handling of the high currents necessary; the resulting lack of homogeneity particularly where those alloys used have no single component which constitutes at least 90% of the overall alloy material. The wide variety of alloys which have been proposed for use as the reactive material in hydrogen storage electrodes are alloyed extensively and are therefore prone to exhibit non-homogeneity. In addition, the high cost of this process both in terms of high energy and high labor costs results in process inefficiencies.
As stated by Michael A. Fetcenko in U.S. Pat. No. 5,002,730, oxygen is a contaiminate in the metal alloys used as electrodes in rechargeable electrochemical batteries. Based on heats of formation of oxides and relative stabilities of metal oxides, the oxygen is expected to be present primarily as zirconium oxide which decreases the amount of zirconium available as active hydride forming sites which will decrease the storage capacity of the cell. Carbon is also considered to be a contaminant since it will also preferentially form compounds with zirconium that decreases the number of active zircomium sites.
While the use of pre-alloys or master-alloys is not new per se it is appreciated by those skilled in the art, that in order to insure complete dissolution of the elemental metal additions, extremely high temperatures are needed. This is particularly so where high melting temperature metals such as Cr are used since that element has a melting temperature of 1857.degree. C. It has been required to heat for 55 to 65 minutes in order to achieve a temperature of about 1800.degree. C. to dissolve the Cr. Additional time is needed to maintain a decreased temperature of 1750.degree. C. for pouring molten metal into a mold. It would be highly desirable to form useful alloys having homogeneity of structure without the use of such high temperatures.
Also since it is undesirable to have methane gas formation during cycling of the rechargeable batteries, it would clearly be an advantage to produce alloy material suitable for secondary battery use having a carbon content sufficiently low so that the alloy when used as an electrode material did not allow significant methane gas to form when cycling the battery.
Various patents relating to hydrogen storage hydride electrode materials methods for preparation and their use in electrochemical applications are already known. Those methods, particularly those for fabricating hydrogen storage electrodes from alloys other than the novel metal alloys and novel processes of the present invention, are incorporated herein and made a part hereof and include U.S. Pat. No. 5,002,730.
It is therefore one object of this invention to produce V-Ni-Cr-Zr-X alloyd having carbon and oxygen contents consistently lower than those of existing battery alloys.